<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <script>
    function loadXMLDoc()
    {
      var xmlhttp;
      if (window.XMLHttpRequest)
      {
        //  IE7+, Firefox, Chrome, Opera, Safari 浏览器执行代码
        xmlhttp=new XMLHttpRequest();
      }
      else
      {
        // IE6, IE5 浏览器执行代码
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
      }
      xmlhttp.onreadystatechange=function()
      {
        if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {
          document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
        }
      }
      xmlhttp.open("GET","/b",true);
      xmlhttp.send();
    }
  </script>
  <script>
  setInterval("test()",500);
  function test1() {
      $.ajax({
        url: '/b',
        async:true,
        type: 'get',
        success: function (data) {
          var new_url =  $('#new_iframe').attr('src');
          if (new_url !== data){
            $('#new_iframe').attr('src', data);
          }
        }
      })
  };
  function test() {
    var n = 1;
    console.log("轮询"+n++);
  };
  </script>
</head>
<body>

<div id="myDiv"><h2>使用 AJAX 修改该文本内容</h2></div>
<button type="button" onclick="loadXMLDoc()">修改内容</button>

</body>
</html>